The Public Transport Authority has a long-term plan to close a stretch of William Street between Hay and Murray Streets to all cars.

Under the plan, only public transport, namely buses, would be allowed on that section of the city thoroughfare.

The proposal would require Barrack Street to become two-way to provide an alternative northern route.

The plan requires the approval of the City of Perth.

Opposition spokesman Ken Travers says it is an ad-hoc decision.

"I think it's another example of the chaotic planning of the Barnett Government, they simply don't have a master plan for how they want Perth to operate in 50 to 100 years with respect to public transport and transport in general," he said.

The Government's proposed light rail system is also expected to run down William Street.

The Premier Colin Barnett says the Government continues to work with the City of Perth on three, shortlisted CBD routes for the proposed light rail system.

The ABC understands the Department of Transport is likely to push for the so-called couplet option, which will put tracks down both the Hay and Murray Street Malls.

The City of Perth is campaigning against light rail down the malls and wants it to run down the Terraces.

The Premier Colin Barnett acknowledges the malls are narrow.

"In many cities around the world now you have light rails operating very efficiently, very safely with both vehicles and pedestrian traffic so it's not beyond the wit of mankind to do this without in anyway harming the city," he said.

The Government will make the final decision on the CBD route after it gets the recommendation from the Department of Transport.